Why seekg function doesn't work?

Question

Here's the code:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main(int argc, char *argv[])
{
  string infile(argv[1]);
  ifstream fin(infile.data());

  string var_name;
  char ch = fin.get();
  cout << ch << endl;

  ch = fin.get();
  cout << ch << endl;

  ch = fin.get();
  cout << ch << endl;

  cout << "pos: " << fin.tellg() << endl;
  fin.seekg(-sizeof(char),ios::cur);
  cout << "pos: " << fin.tellg() << endl;

  ch = fin.get();
  cout << ch << endl;

  return 0;
}

the file content is just a string:

<
?
x
m

and the output is:

<\n
?\n
x\n
pos: 3\n
pos: 2
x

Why the last character printed is still 'x' ?? Why doesn't seekg function move the file pointer back for one byte?

score 4 · Accepted Answer · answered Feb 07 '13 at 13:59

The position of the file pointer was 3 after reading the x, but the x itself is located at position 2 (since the very first character is at position 0). Moving back by 1 character will put the file pointer at the character it most recently read, which is exactly what's happening here.

If you want to move to the character immediately before the last character read, you need to seek by -2, not -1.

score 2 · Answer 2 · edited May 23 '17 at 12:18

2

It will work if you do this: fin.seekg(-sizeof(char)-1,ios::cur);

Note: seeking to an arbitrary location in a text file is Undefined Behavior. See here: How to read the 6th character from the end of the file - ifstream?

edited May 23 '17 at 12:18

Community

1
1

answered Feb 07 '13 at 14:03

Aniket Inge

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`sizeof(char)` is always 1. You might as well just write -2. – Alex Feb 07 '13 at 14:18
That was for completeness :-P @Alex . The OP would understand if I write it this way because he has tried writing it like `-sizeof(char)`. – Aniket Inge Feb 07 '13 at 14:20
1

Then it should be `-sizeof(char)*2` :) – Alex Feb 07 '13 at 14:31

Why seekg function doesn't work?

2 Answers2